1. Field of the Invention
The present invention relates to composite sintered shapes of silicon carbide having a surface layer abundant in rare earth oxides, i.e. a surface layer containing more quantity of rare earth oxides than the interior, and method of producing such shapes. More particularly the present invention relates to composite sintered shapes of silicon carbide having a surface layer abundant in rare earth oxides of insulating property and an interior structure abundant in SiC and having a high bending strength and semi-conductive property, and method of producing such shapes.
Silicon carbide has been used for heat resisting shapes or high temperature heating element due to its excellent high temperature strength, high thermal shock resistance, acid resistance, anti-wearing property and creep resistance, and recently extended its usage to the field of high temperature construction material or anti-wearing material.
However, silicon carbide (hereinafter sometimes referred to as SiC) is generally to be sintered with difficulty, and it is infeasible to produce dense sintered shapes of high strength using SiC alone. Therefore, there have been practised by the hot pressing process wherein a sintering assist such as Al.sub.2 O.sub.3, iron oxide, or AlN is added to SiC powder and mixed together, and the reaction sintering process wherein a shape of mixed powder of SiC and C is melted or reacted with gaseous silicon. However, the conventional processes involve difficulty to form the shapes of complicated configuration and not suitable to mass production. Recently, the most suitable process, pressureless sintering method has been adopted for the manufacture of SiC shapes. This method involves, as disclosed in U.S. Ser. No. 4,090,735, the steps of addition of C and B as sintering assists to SiC powder, mixing the combined powders, forming the mixed powder into green shapes and sintering the same without an application of pressure.
We invented a novel method of manufacture of silicon carbide sintered shapes with or without an application of pressure using specific sintering agents or assists of oxides as disclosed in Japanese Patent Application No. 56-044109, and U.S. Ser. No. 616,787.
In the above-mentioned prior invention, sintering assists selected from the group of rare earth oxides, and further containing one or more members selected from the group consisting of carbon, aluminum oxide and boron oxide are used as sintering assist. The amount of the assist contained in a sintered shape is 11.300 atomic % maximum for rare earth oxides, or 11.500 atomic % maximum rare earth oxides plus aluminum and/or boron oxide. Silicon carbide sintered shapes produced by the above-mentioned invention contain a rather small amount of sintering assist exsisting in the most part in the grain boundary of the shape which assists feasible sintering.
Main object of the present invention is to provide composite silicon carbide sintered shapes especially suitable for substrates of a high dielectric breakdown strength for electronic circuit having a surface layer abundant in rare earth oxides. The object may be achieved by the method defined in the appended claims.